Traditionally, local area networks (LANs) include computer systems configured with a number of processing devices and a server coupled together by a hard-wired connection. Recently, however, wireless LANs have become more common in the marketplace. Although the concept behind wireless LANs has been known for decades, interest in wireless LANs was limited until the release of the 2.4 GHz (2.4 GHz–2.4835 GHz) unlicensed radio frequency band for industrial, scientific and medical applications (ISM band).
The United States Federal Communications Commission (FCC) requires that products used in the ISM band employ “spread spectrum” technologies. The two most common spread spectrum technologies are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) technologies. The DSSS technology presently dominates the ISM band due to the higher throughput, lower latency, better modulation and increased power efficiency offered.
Spread spectrum techniques spread information content over a wider bandwidth than the frequency content of the original information, thus providing a relatively secure form of information transmission. Radio frequency (RF) transceivers employing spread spectrum technologies are well-known in the art and widely used. Although the applications in which spread spectrum transceivers are used are too numerous to describe in detail, increasingly popular applications are in the fields of wireless telephony and wireless computer systems.
A distinguishing feature of spread spectrum technologies such as DSSS is that the modulated output signals occupy a much greater transmission bandwidth than the baseband information bandwidth requires. The spreading is achieved by encoding each data bit in the baseband information using a codeword, or symbol, that has a much higher frequency than the baseband information bit rate. The resultant “spreading” of the signal across a wider frequency bandwidth results in comparatively lower power spectral density, so that other communication systems are less likely to suffer interference from the device that transmits the spread spectrum signal. It also makes the spread signal harder to detect and less susceptible to interference (i.e., harder to jam).
Although the DSSS technology brings these benefits to a wireless LAN, another key performance parameter of any communication network, particularly computer networks and cellular telephone systems, is the available transfer rate of data between devices in the communication network. Wireless LANs are no exception. It is therefore important to maximize the rate at which data may be exchanged between transmitters and receivers in a wireless LAN in order to maximize overall LAN performance.
At present, perhaps the most popular application in the ISM frequency band is the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, which is compatible with the DSSS technology in a wireless LAN. IEEE 802.11 is a set of specifications intended to create a standard for wireless local area networking technology. These rules have been under development for several years and have evolved into a rather broad set of specifications for both direct sequence and frequency hopping spread spectrum systems. The current frequency bandwidth of the 802.11 DSSS wireless LAN in the ISM band is fixed at 22 MHZ and supports only four data transfer rates: 1) binary phase shift keying (BPSK) at 1 Mbps with 1 bit per symbol; 2) quadrature phase shift keying (QPSK) at 2 Mbps with 2 bits per symbol; 3) complementary code keying (CCK) at 5.5 Mbps with 4 bits per symbol; and 4) complementary code keying (CCK) at 11 Mbps with 8 bits per symbol. Those skilled in the art understand the limitations involved when only a few transfer rates are available in a band having ever-growing popularity.
Limited to only four data rates, one of the foreseeable problems facing the 802.11 wireless LAN in the ISM band is an increase in interference. An 802.11 DSSS system found in the ISM band typically withstands interference so long as the instantaneous bandwidth of that interference is limited. More specifically, the process gain provided in DSSS wireless LAN systems (FCC Part 15.247 requires a process gain of at least 10 database) by spreading their signals over the 22 MHZ bandwidth of the ISM band allows them to tolerate narrow band interference even if that interference is nearby. However, if interference grows to cover a significant portion of the 22 MHZ bandwidth, or if the strength of the interference signal is extremely intense, then the performance degradation caused by the interference will become significant.
Currently, the free and unlicensed 2.4 GHz ISM band is very appealing to companies in the wireless field and has experienced rapid growth in the number of products used in the band. As a result, excessive mutual interference is inevitable. For example, many 2.4 GHz cordless telephones occupy significant bandwidth (up to several MHZ in bandwidth) which may impact the performance of the 802.11 wireless LAN.
Another foreseeable problem facing the 802.11 wireless LAN is compatibility with adjacent frequency bands, such as the multi-channel multi-port distribution service (MMDS) band. As only one example, the MMDS band (2.5 GHz–2.686 GHz) is a licensed band whose more regulated use may become overwhelmingly attractive to wireless companies tired of facing congestion problems in the unlicensed ISM band. A fixed bandwidth of 6 MHZ in the MMDS band, much smaller than the fixed 22 MHZ of the ISM band, may also present compatibility problems for 802.11 applications set for only 22 MHZ operation.
Yet another problem is the currently limited data rates (presently only the four data rates listed above) of the ISM frequency band. The fixed 22 MHZ bandwidth in the ISM band limits the number of available channels that may be used by an 802.11 DSSS system in a single area. Moreover, the limited data transmission rates may not be utilizing the precious bandwidth efficiently by occupying large amounts of bandwidth when not necessary.
Still a further problem facing the 802.11 wireless LAN is the interest shown by Internet Service Providers (ISPs) to provision the ISM band for broadband data service. Since the IEEE 802.11 DSSS wireless LAN is the most popular application in the ISM band, the 802.11 system must cooperate with provisioning schemes if the ISPs choose to incorporate the ISM band, or any other band in which the 802.11 wireless LAN may operate, into future broadband data service.
Accordingly, what is needed in the art is a controller for wireless communications networks (including those incorporating spread spectrum systems) that take into account the aforementioned challenges and overcome the deficiencies in the prior art.